Problem at highway speeds
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RWD 19xx-1984 DeVille and Fleetwood,
1985-1996 Fleetwood and Brougham Forum Discussion, Problem at highway speeds in Past Cadillac Vehicle Discussion; I have a 1993 Fleetwood Brogham with 73,000 miles. I have replaced the o2 sensor, throttle position sensor, IAC valve, ...
  1. #1
    fireman130's Avatar
    fireman130 is offline Cadillac Owners Member
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    Question Problem at highway speeds

    I have a 1993 Fleetwood Brogham with 73,000 miles. I have replaced the o2 sensor, throttle position sensor, IAC valve, plugs, cap and rotor, and wires. We checked the egr valve it seems to be working fine and also cleaned it. When the car hits highway speeds, usually over 45 mph it starts to buck and surge and run like crap. If I take it out of overdrive and RPM's are up a little the car runs fine, it seems like when the RPM's are low it really acts up. Anybody have any ideas. The only thing I can think of is the EGR, when we had it off the baffle holds suction if you put your finger over the port and hold it. Thanks for any help.

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    csbuckn is offline Cadillac Owners Master
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    Torque converter not locking up. Not sure of how to fix it.

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    sven914 is offline Cadillac Owners Connoisseur
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    Re: Problem at highway speeds

    Check or clean the Mass Air Flow (MAF) sensor.

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    jayoldschool is online now GM RWD V8 addict
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    Re: Problem at highway speeds

    Drive in OD at the speed where the problem is. Flat ground. Keep your right foot on the gas. Lightly apply the brakes with left foot, maintaining your speed by applying more throttle as needed. What happens?

  6. #5
    fireman130's Avatar
    fireman130 is offline Cadillac Owners Member
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    Re: Problem at highway speeds

    If I hold my foot on the brake a little and throttle up to maintain the speed it stumbles for a second then clears up and runs fine. We checked the fuel pressure while driving it maintained a 13-15 psi even when the car was acting up. Thank you for the help, this is my first nice big ole Caddy and it's turning into a nightmare, I thought by buying it off of a funeral home it would run for a long time, without many issues.

  7. #6
    csbuckn is offline Cadillac Owners Master
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    Re: Problem at highway speeds

    Did you replace all those parts trying to figure out this problem? The test Jay just had you do proves its the torque converter. You torque converter is supposed to lock up when you reach cruising highway speeds to give you better gas mileage, when you hit the brakes, its supposed to unlock. Now I would google "torque converter not locking up on 700R4" and see what it says. I'd bet you just need to drop the tranny pan and replace the solenoid. There is a connection on the left side of the tranny to check also. Probably a fuse for it also but that would suck if it was just a fuse. On another note, I bet the car drives pretty good with all the new parts(besides the TC lockup).

  8. #7
    sven914 is offline Cadillac Owners Connoisseur
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    Re: Problem at highway speeds

    The torque converter not locking up wouldn't cause the driveability concerns listed. If the converter doesn't lock up, it causes the engine RPM's to slightly increase, the transmission to run hot, and poor fuel mileage. There's nothing that should cause the engine to surge or buck.

    The converter may be stuck in the engaged position, which would cause bucking when the transmission shifts out of 3rd (direct drive), but it wouldn't be constant at any speed over 45 mph; it would more or less be a shutter feeling when slowing down and a hesitation when accelerating.

    Engine RPM's directly correlates to the amount of air coming into the engine. The MAF sensor may not be able to properly measure the air flow below a certain point, which would cause the air/fuel mixture to be incorrect and the engine to run to run like crap. The MAF sensor should be located directly behind the air cleaner, and easily accessible for cleaning or checking.

    Here is video on how to clean a MAF sensor:


  9. #8
    fireman130's Avatar
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    Re: Problem at highway speeds

    Does the 5.7 throttle body have a MAF sensor? Where would I find this transmission fuse, I am open to any kind of help or thoughts at this point. Now the engine is surging at idle that's a new problem that started with all these mew parts........

  10. #9
    csbuckn is offline Cadillac Owners Master
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    I'm about 95% sure it's the tc trying to engage which was why Jay recommended the brake test. The tc is continually trying to engage which is causing the shaking. Applying the brake tells the solenoid to disengage the tc which is why it smoothed out when you hit the brakes. The fuse would be labeled trans/ga but I would check them all because they are funny like that. I don't think the 93 has a mass air flow sensor.

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    Re: Problem at highway speeds

    I did the break test tonight. I could watch as soon as the brake lights came on the RPM'S went up and the car worked fine. Is there a fuse or relay someplace I can check for the torque converter, or is it all internal in the transmission? And yes, my friend is a weekend warrior mechanic we replaced all of those parts. Thank you all for the help.

  12. #11
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    Re: Problem at highway speeds

    No MAF on the TBI cars. The problem is lock up related. I'm thinking torque converter lock up solenoid. As Sam said, check all fuses first! Check fluid level, too. THEN, check thermostat operation. Speaking from personal experience, the TBI cars don't like to lock up correctly with cold trans fluid caused by a stuck open stat.

  13. #12
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    fireman130 is offline Cadillac Owners Member
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    Thanks for the reply. I'm starting to lean towards the solenoid also. I checked the fuses and fluid level, I honestly don't know if the transmission was ever serviced being a funeral car it really didn't go too fast or far. But if I put my foot on the brake just enough to make the RPM'S go up it works great. I'm hoping its just the solenoid. There is a local transmission shop right down from my house, I guess that's my next stop.

  14. #13
    jayoldschool's Avatar
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    Re: Problem at highway speeds

    Do research first. If you're handy, you can likely fix it yourself.

    Looks like this, costs $30:



    Drop pan, pull filter, two bolts, one electrical connector:


  15. #14
    fireman130's Avatar
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    Thanks, I'm going to do some searching tonight on how to do it.

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    jayoldschool's Avatar
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    Re: Problem at highway speeds

    Scroll up, I added info!

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